There is a known technology of acquiring a road image ahead a vehicle with a camera mounted on the vehicle, detecting a traffic lane on which the host vehicle is driving, by image processing, and determining a possibility that the host vehicle will deviate from the traffic lane, based on the detected traffic lane information and an estimated path of the host vehicle (e.g., cf. Japanese Patent Application Laid-Open No. 7-105498).
This technology is, for example, to determine a point where the vehicle will deviate from the traffic lane, based on an offset, a yaw angle, a curve radius, etc. of the traffic lane and a yaw rate, a steering angle, a vehicle speed, etc. of the host vehicle, and to predict a deviation state on the basis of a distance between this point and the host vehicle and an angle between the estimated path at the point and a lane line of the traffic lane.